As a complex functional device in integrated optics, an optical resonant ring filter is widely used in optical sensing and optical communication field, and is a core functional device in apparatuses and systems such as lasers, filters, amplifiers, optical switches, and optical delay lines, etc. At present, in the domain of integrated optics, a resonant ring mainly consists of an optical waveguide, and utilizes multiple-beam interferometry to produce a periodic transmitted spectrum and thereby forms a resonant ring filter. For a resonant ring filter, filtering bandwidth, resonance depth, and resonance frequency are crucial performance parameters, and usually the three above parameters must be continuously tunable so as to meet different functions and demands, wherein, the coupling ratio of coupler is related with resonance depth, the intra-ring transmission loss is related to filtering bandwidth, and the intra-ring transmission phase is related to resonance frequency. For a resonant ring in optical waveguide structure, a Mach-Zehnder coupler can be integrated in the ring to regulate the coupling ratio, so as to control the resonance depth; the intra-ring transmission phase can be regulated by means of the thermo-optical properties or electro-optical properties of the optical waveguide material, so as to control the peak value position of resonance frequency. Whereas, for an optical waveguide device, once the device is produced, its structural parameters are set, namely the intra-ring loss and the relevant resonance bandwidth are set and can't be adjusted continuously. Thus, it is difficult to produce a filter in a passive optical waveguide resonant ring structure which can control filtering bandwidth, resonance depth and resonance frequency accurately at the same time. To achieve tunable resonance bandwidth, a technical solution was provided to produce active optical waveguide resonant ring from Erbium-doped optical waveguide and utilize pump light to amplify the signal light and to reduce the intra-ring transmission loss and thereby reduce the resonance bandwidth and increase the definition of resonant ring. However, since such an optical waveguide resonant ring filter in an active structure requires introduction of both pump light and signal light, it increases crosstalk between signals, reduces stability of the devices, and can only modulate signals near 1550 nm waveband; in addition, it can't increase the intra-ring loss and achieve the function of increasing resonance bandwidth.
In recent years, as the nanometer optoelectronic technology progressed continuously, a brand-new waveguide structure—Surface Plasmon Polaritons (SPPs) waveguide became a new trend of research in the field of integrated optics. SPPs are electromagnetic modes produced by interaction between light waves and migratory surface charges (e.g., free electrons in metal), which can achieve transmission in a waveguide made of metal and dielectric materials in a specific structure. SPPs signals can be transmitted a long distance by optimizing the structural design of metal waveguide. The SPPs waveguide has many features that are not available in an optical waveguide, such as signal transmission at nanometer level, which is much lower than optical waveguide level; with the metal waveguide structure, not only optical signals but also electrical signals can be transmitted by SPPs waveguide, making opto-electronic hybrid systems possible; since the characteristics of SPPs mode depend on the refractive index of the dielectric material, the transmission characteristics in SPPs mode can be flexibly controlled by means of the electro-optical and thermo-optical properties of the dielectric material to achieve efficient and tunable SPPs waveguide devices, which is one of the major advantages of SPPs waveguide devices over optical waveguide devices. Various new functional devices based on SPPs waveguide, such as SPPs directional couplers, SPPs waveguide modulators, SPPs waveguide Mach-Zehnder couplers, and SPPs waveguide attenuators, are reported in related experiments, and can be applied in many fields such as optical communication and optical sensing systems, etc.